Apparatus for rough and fine grinding of spherical surfaces



J. R. TURNER June 17, 1952 APPARATUS FOR ROUGH AND FINE GRINDING OF SPHERICAL SURFACES Filed April 20, 1949 2 SHEETSSHEET 1 JOHN R. TURNER INVENTOR M9. (7%

ATTORNEYS June 17, 1952 J. R. TURNER 2,600,815

APPARATUS FOR ROUGH AND FINE GRINDING OF SPHERICAL SURFACES Filed April 20, 1949 2 SHEETSSHEET 2 42 M JOHN R. TURNER" INVENTOR 41 Patented June 17, 1952 APPARATUS FOR ROUGH AND FINE GRIND- ING OF SPHERICAL SURFACES John R. Turner, Rochester, N. Y., assignor to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey Application April 20, 1949, Serial No. 88,624

11 Claims. 1

This invention relates to grinding a spherical surface on a piece of work and particularly to grinding a spherical surface on a plurality of lens blanks.

In the art of lens grinding, it is common practice to start with a lens blank which is molded. or otherwise fabricated, to oversized dimensions, then rough grind the surface to approximately the final or desired curvature, then fine-grind the rough-ground surface sufliciently close to the desired curvature so that the polishing operation will bring the surface to test. Using the conventional technique, this procedure has involved the use of two separate grinding'spindles and tools, chucking the work two separate times. once in relation to each tool, and a separate feeding operation for each of the two tools. Accordingly, the rough and fine grinding of a P ce of work has required two separate Op r ions involving the use of two pieces of apparatus, plus a considerable handling of the work which, in itself, is time-consuming,

One object of the present invention is to provide an apparatus for rough and fine grinding a spherical surface on a piece of work which involves only a single chucking operation of the work relative to the tool, and requires but a single grinding machine and a single feeding operation of the tool, In other words, both the rough and fine grinding operations are carried out successively on a single machine with only one blocking of the work and without requiring any change of tools.

Another object is to provide an apparatus for rough and fine grinding a spherical surface on a plurality of lens blanks in which the two grinds are successively carried out by feeding a pair of concentrically related and rotating annular grinding surfaces. one of the coarse abrasive and the other of fine abrasive, into engagement with the surface of the work and along the axis of the grinding surfaces which is inclinedto the axis of rotation of the work as determined by the curvature of the surface to be generated.

A further object is to provide an apparatus for rough and fine grinding a spherical surface of the type described which is applicable to finishing both concave and convex spherical surfaces.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its methods of op ration, together with additional objects and advantages thereof; will best be understood from the following description when read in connection with the accompanying drawing, in which Fig. 1 is a vertical sectional view showing an apparatus for generating a spherical convex surface on a plurality of lens blanks and constructed in accordance with a preferred embodiment of the present invention;

Fig. 2 is an enlarged sectional detail view showing the relative positions assumed by a new grinding wheel constructed in accordance with the present invention and the original surface of a piece of work to be ground;

Fig. 3 is a'view corresponding to Fig. 2, but showing the relative positions of the wheel surface and the work after the tool has been fed toward the work to bring its surface down to approximately the finished dimension. and showing how the outer ring of coarse abrasive fails to touch the surface when it is ground down to this radius;

Fig. 4 is a view corresponding to Fig. 2, but showing how only the inside edge of he fine abrasive ring on the wheel remains in engagement with the piece of work at the time its surfaceis reduced to the desired curvature;

Fig. 5 is an exaggerated sectional detail of a tool which has been used at least once, and showing how the surfaces of the coarse and fine abrasive rings thereof are worn to different curvatures. This tool is also shown in a position it would assume relative to the surface of a new piece of work and showing that at this time the surface of the fine abrasive ring does not contact the surface to be r Fig. 6 is an enlarged sectional detail of another embodiment of grinding wheel having a. coarse abrasive ring which is much narrower than the fine abrasive ring so as to provide a wheel having a longer life;

Fig. 7 is a vertical sectional view showing how the present invention can be used to grind concave spherical surfaces;

Fig. 8 is a vertical sectional view showing another embodiment of the present invention; and

Fig. 9 is a view taken substantially on line 9--9 of Fig. 8.

Like reference characters refer to corresponding parts throughout the drawing.

Briefly, according to the present invention, the work to be ground is mounted on a block so as to be rotated about an axis which passes through the center of curvature of the surface or surfaces to be generated. A rotating annular or cup-shaped grinding wheel, having its axis of rotation inclined to the axis of rotation of the work as dictated by the curvature of the surface to be generated, is then fed along its axis into engagement with the work. The invention makes use of a special form of grinding wheel employing an inner ring of bonded fine abrasive surrounded by an outer ring of bonded coarse abrasive; the diameter of the inner ring bein great enough so that its periphery embraces all of the surface to be worked. It has been found that the change in radius of curvature of the work, from the beginning of rough grinding to the end of fine grinding, is sufficient to cause such a grinding wheel to begin to grind along its outside edge and to finish grinding toward its inside edge. Thus, the entire roughand finished grind of a complete block of lenses may be accomplished by a single infeed of the generating wheel and a single chucking of the block of lenses.

Referring now to the drawings, in Fig. l I have shown a preferred form of apparatu for rough and fine grinding, a convex spherical surface on a plurality of lens blanks in accordance with the present invention. As is the customary procedure, the lens blanks II) to be worked are disposed in seats II around the convex spherical periphery of a blocking tool I2. As is well known, these lens blanks are initially molded, or otherwise fabricated, so that their surfaces to be generated are somewhat oversize, or are of a greater curvature than that finally desired, and they are fastened in these seats I I by pitch, with the surface to be generated extending slightly beyond the periphery of the block. In Fig. 1, the generated surfaces of the blanks are shown substantially in the plane of the periphery of the block as they will be when the surfaces are finish ground to the desired curvature, and each finished curve lies on a circle whose center of curvature is at I4 in the body of the block. The block of lenses is continually rotated through its spindle I5 by any suitable means, not shown, and it will be noticed that the axis of rotation of the block passes through the center of curvature I4 of the surface to be generated on the lens blanks.

The grinding tool is in the form of a cupshaped wheel I6 having a spindle I! which is driven by any suitable means to rotate the Wheel. On the end of the grinding wheel, there is fixed an outer ring I8 of bonded coarse abrasive, and an inner ring I9 of bonded fine abrasive. The diameter of the wheel I6 is determined by the radius of the spherical surface to be generated, and is large enough so that the inner abrasive ring I9 will embrace the entire surface to be ground as it rotates relative to the wheel. Or, stating it another way, the diameter of the inner ring will be large enough so that its chord will include approximately half of the blocking angle of the block on which the lenses are mounted. This grinding wheel is located so that its axis of rotation is inclined to the axis of rotation of the block and passes through the center of curvature I of the surface to be generated. Obviously, this angle of inclination of the grindin wheel will vary with the radius of curvature of'the particular surface to be generated. Because of their long wearing quality, rings I8 and I9 of bonded diamond dust have been found very useful in this grinding wheel. However, it will be appreciated that bonded abrasives of a character other than diamonds could be used within the spirit of the present invention.

With both the block of lenses and the grinding wheel rotating about their individual axes, the

wheel is fed toward the work along its inclined axis until the desired fine ground spherical surface is obtained. Any suitable means may be provided for feeding the Wheel along its axis toward the work; for example, air pressure as shown in Fig. '7 and described hereinafter. The sam result could be obtained by holding the grinding wheel stationary and movin the block of lenses toward it along the wheel axis, or by moving the wheel and work toward each other at the same time, so long as their engagement is in a direction along the wheel axis. Since, at

he beginning of the operation the radius of curvature is largest, the outside edge of ring I8 will engage the surface of the lens blanks and start the rough grinding. As the wheel is fed toward the work and the radius of curvatur of the ground surface decreases, the point of engagement between the surface being ground and the rings of abrasive I8 and. I9 will move inwardly of the rings until, as the desired curve is approached, the ring I9 of fine abrasive will alone be contacting the work and the final grind on the surface will be a fine one which brings the surface to test. The wheel spindle may have a micrometer stop arrangement associated therewith, not shown, which will automatically stop the feeding operation of the wheel when a given amount of glass has been removed to bring the fine ground spherical surface down to the desired radius of curvature. Accordingly, a block of lenses is given a combined rough and fine grind with but a single feeding operation of a grinding wheel and with but a single chucking operation of the block of lenses on a single machine. This method, therefore, cuts the time originally required to rough and fine grind a block of lens blanks approximately in half, reduces the amount of handling of the block of lenses to half, and also cuts the machine requirements.

In an effort to clearly show and explain just how and why feeding a grinding wheel of the type described in the manner set forth accomplishes the combined rough and fine grinding operation, I have shown, in Figs. 2-5, exaggerated enlargements of a portion of the wheel and the surface which is to be generatet in different positions of feed of the wheel. In each of these figures I have shown the curve to be worked as a continuous one, rather than a plurality of separate ones, but it will be understood that a large single lens could be generated according to this method and with this apparatus, just as well as a plurality of lens blanks.

In Fig. 2 I have shown a new grinding wheel just contacting the original or unground surface to be generated down to a given fine ground spherical surface of lesser radius. Here, the curved surfaces 20 and 2I of both the abrasive rings I8 and I9 respectively is shown to have the same radius of curvature and both engaging the surface to be generated. As the wheel is fed toward the work, the ring I8 of coarse abrasive will do the major part of the grinding, since the outside edge of the ring I8 engages the work with firmer pressure and the fine abrasive ring I9 wears away faster than the coarse abrasive ring I8. As the wheel is fed to the work and the radius of curvature of the work becomes shorter and shorter, the point of engagement between the work and the surface of the rings I8 and I9 will move inwardly of the rings until finally the outer ring I B is. not contacting the work with any appreciable pressure, but the inner ring I9 of fine abrasive is doing the grinding. I have endeavored to show this condition in Fig. 3 in an exaggerated way. Then, as the wheel reaches the end of its feeding stroke, and the surface being generated has substantially reached test, only the inside edge of ring I9 is engaging the work, and the surface is being brought to the desired curvature with a fine grind operation. This is shown in an exaggerated way in Fig. 4. Accordingly, after a grinding tool has made an initial pass over a block of lenses, then surface 20 of the ring I8 of coarse abrasive will have a given curvature determined by the change in radius of the work, whereas the surface 2 I' of the ring I9 will have a curvature other than that of ring I8 as determined by the change in radius of the work introduced during its engagement with the work. While the curvature of the surfaces 29 and 2| of rings I8 and I9 respectively will not be truly spherical in themselves, because of being determined by a decreasing radius in the work as the tool is fed, they will each have a curvature of different radius, and generally corresponding to the starting radius of curvature of the work and the final radius of curvature of the work, respectively. In Fig. 5, I have designated these two radii of curvatures of the surfaces 29 and 2| as R1 and R2; these radii corresponding to the radii of the work at the start and at the finish of the grinding operation. Looking now at Fig. 5, it will be seen that when a wheel having surfaces on its rings of abrasive having radii R1 and R2 is brought into engagement with a new block of work, the surface of inner ring I9 does not contact the work at all and will not come into engagement with the work until the radius of curvature thereof approaches radius R2. In this figure, as in Figs. 2-4, the reference numeral I generally indicates a new surface to be generated; the numeral 2 indicates the surface after being substantially completely rough ground, and 3 indicates the surface as substantially brought to test. The radii of curvature of surfaces 29 and 2| and the manner in which they successively contact the work as the wheel is fed inwardly, is obviously exaggerated to clearly point out the principles of operation of the present invention. Instead of the surfaces 2|] and 2| of the abrasive rings I8 or I9 having decidedly different radii of curvature which join in a ridge, as shown in Fig. 5, these two surfaces will, in fact, blend together so that their difference in curvature is difficult to detect by feel, or with the naked eye. While I have shown the surface 2|] of the coarse abrasive ring I8 and the surface 2| of the fine abrasive ring I9 definitely spaced from the surface of the work in the final and initial positions of the wheel, respectively, actually, when the surface of ring I8 is firmly engaging the work, the surface 2| of ring I9 may be lightly touching the work, but doing no grinding to speak of. By the same token, at the final position of the wheel, the tips of all or some of the diamonds in the ring I8 may actually touch the surface being ground, but all of the effective abrading action will be done by the surface 2| of the inner ring I9.

While in each of Figs. 2-5 I have shown the rings of coarse and fine abrasive as being substantially equal in width, such a relationship as to size is not necessary and is, in fact, susceptible to wide variation with advantages. For instance, because the wearing qualities of the bonded coarse abrasive is appreciably greater than that of the bonded fine abrasive, I have found that the ring of coarse abrasive can be madequite narrow, as

compared with the width of the ring of bonded fine abrasive, and still accomplish the desired combined rough-fine grind operation. Such a grinding wheel is indicated in Fig. 6 where l8 indicates the ring of bonded coarse abrasive, and I9 indicates the ring of bonded fine abrasive. This relationship of rings I8 and I9 provides a combined rough and fine grind wheel which has a much longer life than one in which the rings of the two abrasives are of equal width.

This method of grinding is applicable to the generation of spherical concave surfaces, as well as spherical convex surfaces. As shown in Fig. '7, for generating a spherical concave surface on a plurality of lens blanks, the blanks I0 would be fixedly located in sea-ts II in the spherical concave periphery of a blocking tool I 2'. This block is then rotated by means of a spindle I5 about an axis which passes through point I4 which is the center of curvature of the finished surface desired on the lens blanks. As before, a grinding wheel I6, which is adapted to be rotated about its spindle axis by any suitable means, not shown, is disposed with its axis inclined to the axis of rotation of the block I2 and passing through the center of curvature I4. Fixed to the end of the wheel IS in side-by-side relation are a ring I8" of bonded coarse abrasive and a ring I9 of bonded fine abrasive. As before, the ring of coarse abrasive being outside of the ring of fine abrasive. In this case, the work-engaging surfaces 20" and 2|" of rings I8" and I9 are convex instead of concave, as in the case for generating convex spherical surfaces. The wheel I6 may be fed toward the work along its axis by any suitable means and the change in radius of curvature of the work from the beginning of rough grinding to the finish of fine grinding is sufficient to cause the ring I 8" to begin to grind along its outside edge and to finish grinding along the inside edge of ring I9", as outlined above. In this instance, the radius of curvature of the work will increase, rather than decrease, as the grinding proceeds, and the surfaces 20 and 2|" of rings I8" and I9" will assume different curvatures which will cause them to work in succession as the wheel is fed to the work in a manner analogous to that described above. For purposes of illustration 13 have shown the wheel-feeding means as comprising a cylinder C in which an enlarged end E on the tool spindle I'I acts as a piston. Air introduced into cylinder 0 through port P from air line A will normally force the wheel I6 against the block of work.

It has been found that the method of combined rough and fine grinding described works particularly well on blocks of lenses of large included angle such as those of 70 blocking angle, or more. This blocking angle referred to is the half angle included between the vertical axis of the block and the radius including the lowest lens on the block. In Fig. 1 this blocking angle is more than while in Fig. 7 it is just under 90. While weak blocks could also be done by this method, these are less feasible, unless a very large diameter grinding wheel is used so as to obtain a grinding angle (the angle between the wheel axis and the block axis above the block) approaching 45. If the curvature of the surface to be generated is such that difficulty is encountered in obtaining a changeover from a rough grind to a fine grind with a single feeding strok of the wheel, it may be necessary to retract the rough grinding element slightly as the grinding proceeds.

One form ofapparatus fordoing is shown in Figs. 8 and 9 wherein a grinding wheel 4'0 having a ring 4 lof bonded fine abrasive is mounted in telescoping relation with a grinding wheel 42 having a ring 43 of bonded coarse abrasive. A spindle 44 on the-wheel 40 has a sliding fit with a sleeve 45 on wheel 42; said sleeve,,in turn, having a sliding and rotating fit with a cylinder 46. Spindle 44 is connected to sleeve 45. by keys and keyways 41 and 48, respectively, so. as to be rotated with sleeve 45 when it isdriven by :a belt 49 engaging pulley 50 fixed to thesleeve. A compression spring 5.! located. between keys 4'! and the wheel 42- normally moves wheel 40 to the retracted position shown and which. position is determined by keys 4! engaging thev top. of the keyway-s in sleeve 45 The top of, cylinder 46. is covered by a cap 52 including a portion 53 telescoping with the upper end of sleeve 45. Sleeve 45 has a reduced diameter 54 roviding a circular chamber 55 between the cylinder wall and thesleeveand which is connected to the interior of the sleeve at a point above the end of the spindle 44' by a plurality of radial ports 55. Thiscircular chamber 5515 connected with an inlet port 57 in the cylinder wall and to which air, under pressure, may be fed from supply line 58o-r from which air may .beexhausted under the control of rotary valve 60.

The block of-lenses is properly located with its axis of rotation inclined .to the rotational axis of wheels 40 and 42, as ab-ovedescribed, and when the valve 60 is rota-ted to the position shown in Fig. 8, air under pressure entering chamber 55 acts .on the top of spindle 44 to. force it downwardly toward the work along the axis. of rotation of the wheels. Because compression spring is fairly heavy, sleeve 45 will beforced downwardly with spindle 44 and wheel 4.2, and its ring 43 will be forced againstthe work. Spindle 44 and sleeve 45' will continueto befed together .in the relative positions shown until adjustable stop nut 5| on the top end of sleeve 45strikes the top of flange 62 within cylinder 46,, whereupon the feeding motion of sleeve 45 will cease. This indicates the endof the rough grind operation and will be selected by adjustment of stop nut 6|. Thereafter spindle 44 will move downwardly toward the work relative to sleeve 45 to bring the fine abrasive ring 4] on wheel into engagement,

with the work. Spring 51 compresses to allow this movement of the spindle relative to, the sleeve. Spindle 44 will continue to move downwardly under thea-ction of the air pressure until the fine grind operation is completed and the surface being generated reaches test. This may be determined by a positive stop. such as complete compression of spring 54, and, at this point, the valve 60 will be rotated to exhaust, air from above the spindle and place the air supply in communication with circular chamber 65 formed in the lower end of the cylinder through port 66. This chamber is closed at the bottom by a gasket t1 fastened to thebottom of the cylinder and engaging the sleeve with a fairlytight sliding and rotating fit. Air, upon entering this chamber, presses upwardly against flange 58 on sleeve 45 to force the sleeve and spindle 44to araised position for the next operation.

By this arrangement of parts, the rough and fine grinding wheels are brought into successive operation with a block. of lenses by feeding the two wheels along their common axis relative to the work, and both the, rough and fine grind operations are performed on one machine with a single feeding operation of the wheels, andwith but a single chucking operation of the work, whereas in the first embodiment the successive operation of the rough and fine grind rings depends upon the change in radius of the work. With this last-mentioned embodiment, the successive operation of the rough and fine grind rings is under mechanical control so that the'desired successive action of the two different grades of abrasive rings can be obtained regardless of the change in curvature which the work is susceptible of during the complete generating operation.

Although I have shown and described certain specific embodiments of my invention, I am fully aware that many modifications thereof are possible. My invention therefore is not to be limited to the precise details of construction shown and described, but is intended to cover all modifications coming within the scope of the appended claims.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent of the United States is:

1. An abrading tool for generating a spherical surface on a piece of work comprising a pair of telescopically arranged sleeves adapted to be rotated simultaneously about their common axis, a ring of bonded coarse abrasive fixed to the end of the outer sleeve, a ring of bonded fine abrasive fixed to the end of the inner sleeve, the exposed face of each of said rings being arcuate and having substantially the same center of curvature, and means for telescoping one sleeve relative to the other to extend and retract the face of one of said ring-s axially relative to the face of the other.

2. An abrad-ing apparatus for rough and fine grinding a spherical surface on a piece of work with one feeding stroke of a single tool and comprising in combination arota-table work spindle, means for blocking said work on said spindle in such a way that the center of curvature of the surface to be'generated will be on the axis of said spindle, a rotatable cup-shaped grinding wheel.

disposed with its axis of rotation inclined to the axis of said work spindle and intersecting the center of curvature of the surface to be generated, the rim of said grinding wheel comprising a ring of bonded coarse abrasive and a ring of bonded fine abrasive connected in concentric relationship with the ring of bonded fine abrasive lying within the other ring the exposed ends of said abrasive rings constituting the work-engaging faces and being arcuate about centers lying on the axis of said wheel, and means for feeding said grinding wheel into engagement with said piece of work along the axis of said wheel until the desired fine ground surface is, obtained on thepiece of work.

3; An abradlng apparatus according to claim 2 inwhich the width of the working face of the ring of bonded coarse abrasive is only a fraction as wide as theworking face of the ring of bonded fineabrasive.

4. An abrading tool for generating a spherical surface on a pieceof work comprising a cupshaped wheel adapted to be fed axially toward the surface to be abraded; a ring of bonded coarse abrasive fixed to the endof the rim of said wheel, and a ring of bonded fine abrasive fixed to the end-of the rim of saidwheel within, and in adjacent concentric relation to said ring of bonded coarse abrasive, the exposed ends of said abrasive rings constituting the work-engaging faces and being arcuate about centers lying on the axis of said wheel, said inner ring having a diameter sufficiently large that its periphery will fully embrace the spherical surface to be ground when said wheel is moved into engagement with said surface alone, the wheel axis when said axis is inclined to the axis of rotation of the work and passes through the center of curvature of said spherical surface.

5. An abrading tool for generating a spherical surface on a blank of work initially having a generally spherical surface of radius of curvature different than the radius of curvature of the surface to be generated on the work, and comprising a cup-shaped wheel, a ring of bonded coarse abrasive fixed directly to the end of the rim of said wheel, and a ring of bonded fine abrasive fixed directly to the end of the rim of said wheel within, and in adjacent concentric relation to, said ring of coarse abrasive, the exposed ends of said rings constituting the work engaging surfaces and being arcuate and substantially contiguous and defining a sphere having a radius of curvature substantially equal to that of the blank of work prior to grinding.

6. An abrading tool according to claim in which the exposed arcuate end of the outer ring has a radius of curvature substantially equal to the radiusof curvature of the blank of Work prior to grinding and the arcuate end of the inner ring has a radius of curvature substantially equal to the radius of curvature of the surface to be finally generated on the blank of work.

7. An abrading tool according to claim 4 in which the radial width of the ring of coarse abrasive is narrower than the radial width of the ring of fine abrasive.

8. An abrading apparatus according to claim 2 characterized by the fact that the work-engaging faces of said rings of abrasive are'arcuately shaped to a different degree so that during the initial part of the grinding operation the ring of coarse abrasive engages the work and when the radius of curvature of the work approaches the desired radius, the ring of fine abrasive automatically takes over the grinding operation from the ring of coarse abrasive and continues to grind until the desired surface is generated on the work.

9. An abrading apparatus for rough and fine grinding a spherical convex surface of given radius of curvature on a piece of work initially having a spherical convex surface of larger radius of curvature than finally desired and comprising in combination a rotatable work spindle, means for blocking said work on said spindle in such a way that the center of curvature of the surface to be generated will be on the axis of said spindle, a rotatable cup-shaped grinding wheel disposed with its axis of rotation inclined to the axis of said work spindle and intersecting the center of curvature of the surface to be generated, the rim of said grinding wheel having fixed thereto a ring of bonded coarse abrasive and a ring of bonded fine abrasive, the ring of fine abrasive being disposed concentrically within, and in abutting relation with, said ring of coarse abrasive, the exposed ends of said rings having spherically concave surfaces which are contiguous and together define a spherical concave surface which is substantially complementary to the spherical convex surface on the piece of work as blocked, and means for feeding said grinding wheel into engagement with said piece of work along the axis of said wheel until the desired fine-ground surface is obtained on the piece of work.

10. An abrading apparatus for rough and fine grinding a spherical concave surface of a given radius of curvature on a piece of work initially having a spherical concave surface of smaller radius than that finally desired and comprising in combination a rotatable work spindle, means for blocking said work on said spindle in such a way that the center of curvature of the surface to be generated will lie on the'axis of said spindle, a rotatable cup-shaped grinding wheel disposed with its axis of rotation inclined to the axis of said work spindle and intersecting the center of curvature of the surface to be generated, the rim of said grinding wheel having fixed thereto a ring of bonded coarse abrasive and a ring of bonded fine abrasive, the ring of fine abrasive being disposed concentrically within, and in abutting relation with, said ring of coarse abrasive, the exposed ends of said rings having spherically convex surfaces which are contiguous and together define a spherical convex surface which is substantially complementary to the spherical concave surface on the piece of work as blocked, and means for feeding said grinding Wheel into engagement with aid piece of work along the axis of the wheel until the desired fine-ground surface is obtained on the work.

11. An abrading apparatus for rough and fine grinding a spherical surface on a piece of work and comprising in combination a rotatable work spindle, means for blocking said work on said spindle so that the center of curvature of the surface to be generated will coincide with the axis of said spindle, a rotatable tubular grinding tool disposed with its axis of rotation inclined to the axis of said work spindle and intersecting the center of curvature of the surface to be generated, a ring of bonded coarse abrasive fixed to the end of said tool and having an arcuate abrading face forming a part of a sphere substantially complementary in shape to the spherical surface on the work when blocked, a second rotatable grinding tool telescopically disposed within said first tool, a ring of bonded fine abrasive fixed to the end of said second tool and having an arcuate abrading face forming a part of a sphere substantially complementary in shape to the spherical surface to be generated on the work, means normally retracting said second tool relative to said first tool sufficiently to withdraw the ring of fine abrasive thereon from the grinding-plane of the abrasive ring on said first tool, and means for successively feeding the first and then the second tool into engagement with the work to rough and then fine grind the work to generate the desired surface thereon.

JOHN R. TURNER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,327,440 MacGregor "on Jan. 6, 1920 1,401,832 Taylor Dec. 27, 1921 1,828,663 Jopp Oct. 20, 1931 2,286,361 Goddu June 16, 1942 2,309,016 Ryan Jan. 19, 1943 2,451,295 Metzger et a1. Oct. 12, 1948 2,510,113 Holman June 6, 1950 FOREIGN PATENTS Number Country Date 199,312 Germany June 12, 1908 

